The present invention concerns more precisely an automatic gain control (AGC) circuit including:                a variable gain amplifier (VGA) adapted to receive a received signal and to output an amplified signal to an analog to digital converter (ADC),        and a gain controller (GC) which is connected to said variable gain amplifier for receiving said amplified signal and for controlling a gain of said variable gain amplifier.        
When the received signal is coming for instance from a vibration sensor, the received signal can be an impulsive signal with variations in amplitude of several orders of magnitude.
Generally, low input gain and high resolution analog to digital converters (ADC) are used to convert such received signal. But when the received signal is an impulsive signal, it is necessary to tune an important margin between the ADC full scale input voltage range and the received signal maximum voltage. That is why most of time, the amplified signal has a very small amplitude and is noisy. Moreover, the use of high resolution ADC is costly.
Another way is to use an automatic gain control (AGC) circuit in front of the ADC. An AGC refers to a circuit which gain varies substantially depending on the magnitude of the received signal so as to maintain the amplified signal level substantially constant. As described is U.S. Pat. No. 5,606,284, the AGC circuit comprises at least one analog amplifier for the received signal which is automatically controlled by an analog feedback signal. Unfortunately amplified signal must be integrated in the feedback loop to produce the VGA gain control voltage. Thus, time constant must be sufficiently large and such AGC circuit is not well adapted to impulsive signals with lots of variations in amplitudes.